As the cost of fossil fuels continues to increase, efforts continuously are being made to extract energy from the sun as much as possible. One area where this has been done for a number of years is to use solar energy to heat water to provide a supply of hot water for residential or commercial building. Systems for doing this range from relatively simple and somewhat inefficient systems to elaborate, expensive solar water systems which are capable of replacing the conventional gas or electrically heated hot water storage tank. In most cases, it is necessary to employ both a solar system and a conventional hot water tank, with the two systems either being operated in tandem or selectively operated individually depending upon the solar energy available. Solar hot water heating systems, however, are relatively expensive and for the most part are not cost effective as replacements for conventional hot water heating systems.
To overcome some of the high cost disadvantages of solar systems, simpler systems have evolved to operate in tandem with conventional hot water heating systems to preheat the water supplied to the conventional hot water storage tank by first passing it through some type of a solar hot water tank.
A system directed to a selective control of a combination solar hot water heater and a conventional hot water tank is disclosed in the Warden U.S. Pat. No. 4,403,602. This patent discloses a system in which a closed fluid loop is used in conjunction with solar panels for providing a heat source for heating water from a residential water supply or the like in a solar tank. The water which flows into and out of the tank does not itself flow through the solar heating unit. A conventional domestic tank also is employed and a valve arrangement is used to selectively provide all of the hot water heat from the solar heated tank, or all of the hot water heat from the domestically heated tank, or to connect the tanks in tandum where the water from the cold water supply first enters the solar heated tank and then passes from that tank into the conventionally heated tank from which it is drawn for use in the building. Five valves are shown for variously diverting the water through the different paths. The valves themselves are shown as conventional circular handle valves which do not provide any indication of the state of operation of the valve. Secondly, the system requires a pump to recirculate the heating fluid through the solar system into the solar heated hot water tank and no arrangement is made to drain any fluid from that system. The hot water which is being heated for use in the building does not pass through the solar collectors, but the closed loop system including the pump is used for that purpose.
Another patent of interest is the patent to Weatherbee U.S. Pat. No. 2,689,090. The heating system disclosed in the Weatherbee patent includes a set of solar panels on the roof of a building and another heat exchanger coil buried in the ground. There is an automatic switch-over accomplished by means of temperature sensing to cause the fluids circulating through the system to be through the ground alone or through a combination of the ground and solar system. The Weatherbee system is directed to a heating system rather than a hot water supply system, however, so that a closed loop configuration is utilized.
A shared hot water solar system is disclosed in the patent to Geaslin U.S. Pat. No. 4,191,329. Like the Weatherbee patent this is a shared system which connects in series a solar system and a domestic hot water heater for supplying water to the hot water taps in the home. A switch-over automatically is effected to the domestic hot water heater alone whenever the solar temperature is too low. A provision also is made in this patent that whenever the water in the solar panels approaches a freezing temperature, water is circulated from the household loop to the solar panels to prevent them from freezing. In very cold climates, almost continuous circulation of water in this manner would occur. When the system is operated in this mode, it is not an energy efficient system since the warmth in the water is utilized to prevent the system from becoming damaged, and substantial energy of some type must be supplied to that water for this purpose.
Solar hot water systems of various types long have been known. Several old patents directed to a solar system which is connected in series with some type of additional hot water system are the patents to Bentz U.S. Pat. No. 1,849,266; Walker U.S. Pat. No. 735,321; McIntyre U.S. Pat. No. 1,004,888; and Lyon U.S. Pat. No. 4,413,614. None of these patents include any valves or switch-over from one mode of operation to another.
It is desirable, from an energy savings point of view, to utilize as much of the sun's energy as possible. As indicated by some of the above-mentioned patents, this can be accomplished by preheating some of the water for a hot water supply before it enters the conventional hot water tank. For areas of the country where freezing temperatures are encountered, however, it is desirable to provide a simple, inexpensive, and as nearly fool proof method as possible for disconnecting and draining the water out of the solar portion of the system and simply utilizing the standard hot water tank for heating the water. Similarly when freezing is no longer a problem and a solar heating system is capable of contributing at least preheating to the water supply to the hot water tank, the switch-over back to a series or tandum arrangement of the conventional hot water tank and the solar system is desirable.